Related Field
The present invention relates to a shift register, which may prevent current leakage and degradation of an oxide transistor due to light so as to improve output stability, and a display device using the same.
Discussion of the Related Art
Examples of flat display devices include Liquid Crystal Displays (LCDs) using liquid crystal, OLED displays using Organic Light Emitting Diodes (OLEDs), Electrophoretic Displays (EPDs) using electrophoretic particles, etc.
A flat display device includes a display panel for displaying an image through a pixel array in which each pixel is independently driven by a thin film transistor (TFT), a panel driver for driving the display panel, and a timing controller for controlling the panel driver. The panel driver includes a gate driver for driving gate lines of the display panel and a data driver for driving data lines of the display panel.
The gate driver includes a shift register to output scan pulses (also referred to as scan signals) to sequentially drive the gate lines of the display panel. The shift register includes a plurality of interconnected stages, and each stage includes a plurality of transistors. Output of each stage is supplied to each gate line as a scan pulse and supplied as a carry signal for controlling charge and discharge of other stages. Recently, gate drivers mainly use a Gate-In-Panel (GIP) type which is formed together with a TFT array of a pixel array and is embedded in a panel.
Recently, as a TFT for display panels, implementing an oxide semiconductor transistor (hereinafter, referred to as an oxide transistor), which has higher mobility than an amorphous silicon transistor and is more easily applicable to a large area through a lower temperature process than a polysilicon transistor, is proposed. However, the oxide transistor has drawbacks, such as high sensitivity to light (i.e., varying characteristics when light is applied thereto).
FIG. 1 is a voltage (Vg) to current (Ids) graph illustrating variation in a threshold voltage (Vth) according to lapse of time for which light is applied to a general oxide transistor.
With reference to FIG. 1, when light is applied to an oxide active layer of the oxide transistor, threshold voltage (Vth) takes a negative value and, as time elapses, the oxide active layer is degraded due to light, and the threshold voltage (Vth) is further moved in the direction of a negative value.
When the threshold voltage (Vth) of the oxide transistor is moved in the direction of a negative value due to light, leakage current may be increased, and a circuit may be abnormally operated.
In more detail, an N-type oxide transistor is mainly implemented in a shift register and, even if gate off voltage (gate low voltage) is applied as gate voltage to logically turn off the transistor (e.g., Vgs=0V), an overdrive voltage (Vgs−Vth) may be greater than 0V, because the threshold voltage Vth can become negative due to light. Thus, leakage current can flow and the shift register may not output a normal waveform.
For example, when a threshold voltage (Vth) of the oxide transistor has a negative value due to light incident on the oxide transistor, the waveform of a scan pulse output through a pull-up transistor may be distorted or the scan pulse may not be output due to leakage current of a node controller for controlling the pull-up transistor.